Optical information recording media are generally classified into three types which are a read-only type, a write-once type, and a rewritable type based on the principle of recording and reading.
FIG. 1 schematically shows a representative structure of a read-only optical disc. As shown in FIG. 1, the read-only optical disc has the structure in which a reflective film 2 containing Ag, Al, Au, or the like as a main component thereof and a light transmission layer 3 are successively laminated on a substrate 1 of a transparent plastic resin or the like. On the substrate 1, information based on a combination of projecting/depressed lands/pits (recorded data) is recorded, and a substrate made of polycarbonate having a thickness of 1.1 mm and a diameter of 12 cm, e.g., is used. The light transmission layer 3 is formed by sticking a light transmissive sheet or coating/curing a light transmissive resin. The reading of the recorded data is performed by detecting the phase difference or reflection difference of a laser beam applied to an optical disc.
In FIG. 1, there is shown a single-layer optical disc in which the reflective film 2 and the light transmission layer 3 are formed on the substrate 1 on which the information based on the projecting/depressed lands/pits (recorded data) is recorded. However, as shown in FIG. 2, there can be also used, e.g., a double-layer optical disc including a first information recording surface 11 and a second information recording surface 12. Specifically, the double-layer optical disc of FIG. 2 has a structure in which a first reflective film 2A, a first light transmission layer 3A, a second reflective film 2B, and a second light transmission layer 3B are successively laminated on the substrate 1 on which the information based on a combination of the projecting/depressed lands/pits (recorded data) is recorded. In the first light transmission layer 3A, information different from the information recorded on the substrate 1 is recorded based on a combination of pits/lands.
For reflective films used in optical discs, Au, Cu, Ag, Al, and an alloy containing any of these elements as a main component thereof have been used generally heretofore.
Among them, a reflective film of an Au-based alloy containing Au as a main component thereof has the advantages of excellent chemical stability (durability) and a small time-dependent change in recording characteristic, but is extremely expensive. In addition, there is also a problem that a sufficiently high reflectivity cannot be obtained to a blue laser (at a wavelength of 405 nm) used in recording/reading to/from BD or HD DVD. A Cu-based alloy containing Cu as a main component thereof is inexpensive, but poorest in durability among the conventional reflective film materials. In addition, the Cu-based alloy has a drawback of a low reflectivity to a blue laser, similarly to Au, so that the use thereof is limited. By contrast, a reflective film of an Ag-based alloy containing Ag as a main component thereof shows a sufficiently high reflectivity to light in the range of 400 to 800 nm which is a practically used wavelength region, and also has excellent durability. Therefore, the Ag-based alloy is widely used in optical discs using a blue laser.
On the other hand, a reflective film of an Al-based alloy containing Al as a main component thereof is inexpensive and has a sufficiently high reflectivity to light at a wavelength of 405 nm, but has durability inferior to those of Ag type and Au type reflective films. Accordingly, in the case of coating the Al-based alloy reflective film onto DVD-ROM, the thickness of the reflective film is sufficiently increased to a level of approximately 40 nm to improve durability. However, there has been a problem that, when an Al type reflective film having such a thickness is applied to BD-ROM, HD DVD-ROM, or the like which uses a blue laser, the accuracy of a recording signal (reading signal) decreases (i.e., jitter increases), and stable reading cannot be performed.
To enhance the durability and reading stability of the Al-based alloy reflective film, there have been proposed methods shown in, e.g., Patent Documents 1 to 4.
Among them, Patent Document 1 discloses an optical disc having an optical disc substrate formed with pit rows having pits in accordance with a recording signal, a reflective film deposited on the surface thereof where the pits are formed, and a light transmission layer formed on the reflective film. In the optical disc, the pit rows viewed from the light transmission layer include the pits having lengths and widths of 250 nm or less, and the thickness of the reflective film of Al, Ag, or Au is reduced to 20 nm or less. In general, the miniaturization of the pits causes the degradation of signal reading but, in Patent Document 1, the problem of jitter deterioration is circumvented by controlling the thickness of the reflective film to 20 nm or less. However, there is a problem that, when the thickness of the reflective film is reduced to 20 nm or less, sufficient durability in terms of practical use cannot be obtained.
In Patent Document 2, a technology is disclosed which controls pits formed in the surface of a substrate and the space between the pits in association with the length of the substrate, and thereby improves the jitter characteristic of a reading signal.
In Patent Document 3, an optical disc including an Al reflective layer (having a thickness of 100 nm) containing Ta in an amount of 4 percent is disclosed as a read-only optical disc having excellent durability even under conditions in which temperature and humidity abruptly change in the section of Examples.
In Patent Document 4, an Al-based alloy reflective film is disclosed which contains each of Cr, Fe, and Ti in an amount of 1 to 4 percent. By providing an alloy with such a composition, it is possible to obtain a reflective film having a high reflectivity, a flat and smooth surface (having Ra of about 5 to 10 nm), a small growth of crystal grains resulting from a temperature change, and a small reflectivity change.
[Patent Document 1] International Publication Pamphlet WO 00/65584
[Patent Document 2] JP-A No. 66003/2006
[Patent Document 3] JP-B2 No. 62919/1995
[Patent Document 4] JP-A No. 092153/2007